Electroluminescent devices contain phosphor and dielectric films interposed between conductive layers or electrodes at least one of which is transparent. When alternating current is applied the phosphor is activated and light is emitted. The degree of emitted light or brightness depends generally upon the voltage and the frequency of the applied current.
One difficulty with electroluminescent devices is their vulnerability to moisture. Phosphor presents a particular problem because it has a tendency to react with water, including moisture in the atmosphere so that within a relatively short period of time the phosphor degrades and loses its light-emitting capabilities. Traditional approaches to this problem include the coating of the phosphor layer with an impermeable polymer as, for example, a film of chlorotrifluoroethylene or the like; however, the results have not been rewarding.
In U.S. Pat. No. 4,097,776 Allinikov describes a method for producing an electroluminescent cell by coating the phosphor with a liquid crystal to form a water barrier. The phosphor particles are first added to a solution of liquid crystals to form a slurry and the resulting mixture is then stored at temperatures above the liquid crystal melting point. Upon removing the solvent the phosphor particles were found to be encapsulated within the liquid crystal where they are presumably insulated from moisture. However, when added to conventional binders as, for example, binders in which polyvinylbutyral is a principal component the liquid crystals deteriorate and lose their protective ability so that the resulting device fails in relatively short order.
In U.S. Pat. No. 4,513,023 Wary directs attention to the inherent hygroscopicity and sensitivity to moisture of the phosphor component and his understanding of this problem led him to encapsulate the phosphors within a polymeric binder. Specifically, Wary utilizes curable polymeric matrixes as binders and once the phosphor and polymer have been combined the polymer is treated with . ultraviolet (uv) light in an inert atmosphere. The difficulty with this system is the tendency of uv-cured resins to degrade within relatively short operating periods.
In U.S. Pat. No. 4,560,902 Kardon discloses an adhesive-type binder for protecting the phosphor particles from moisture. This binder system is an epoxy type polymer derived from epihalohydrins, bisphenols and urethane resins.
Although the Kardon system represents an advance in the art it too has been found wanting because over a period of time the presence of moisture results in impairment of the phosphor and decomposition of the electrodes.
Accordingly, there remains a need for electroluminescent lamps which possess improved resistance to moisture and which exhibit an improved brightness and longevity when compared to conventional devices.